This invention relates, as indicated, to enzyme electrodes, that is to say electrodes carrying an enzyme immobilized thereon and amperometrically responsive to the activity of that enzyme when the electrode is in contact with a sample, e.g., a clinical sample, containing the enzyme substrate.
In U.S. Pat. No. 4,970,145 enzyme electrodes of improved sensitivity and rapid response time are disclosed and which comprise an enzyme layer immobilised onto an electrically conductive carbon substrate, more particularly an electrically conductive carbon substrate having a heterogeneous porous surface layer onto which the enzyme is immobilised and which consists essentially of finely divided particles of a platinum group metal, e.g. platinum or palladium, uniformly dispersed throughout a porous matrix of resin bonded carbon or graphite particles. Preferred resin binders are hydrophobic resin binders, particularly fluorocarbon resins, especially polytetrafluoroethylene.
The heterogeneous, porous surface layer is preferably formed as a resin bonded surface layer on an underlying electrically conductive support, e.g. a metal sheet, or more preferably, a sheet of electrically conductive carbon paper. Alternatively, the porous surface layer may be formed as a resin bonded surface layer on a filamentous web of electrically conductive fibres, e.g. a filamentous web of carbon fibres.
Alternatively, the heterogeneous, porous surface layer may be an integral, self-supporting layer of resin bonded carbon or graphite particles, having said finely divided platinum group metal substantially uniformly dispersed therethrough.
Whilst the resin bonded carbon or graphite particle layer containing the finely divided platinum group metal can be formed by suitably moulding a uniform mixture containing said carbon or graphite particles and said finely divided platinum group metal, the finely divided platinum group metal is preferably preadsorbed onto the surface of the carbon or graphite particles, prior to the moulding thereof to form said resin bonded layer. Materials made according to that technique, which is disclosed in detail in GB-A-1,357,494, and U.S. Pat. Nos. 4,044,193 and 4,166,143, or in U.S. Pat. Nos. 4,229,490 and 4,293,396 for supported versions thereof, are in fact commercially available materials heretofore used as gas diffusion electrodes in fuel cells, and available from the Prototech Company of Newton Highlands, Mass., United States of America.
Whilst the novel enzyme electrodes described above and disclosed in more detail in the previous application are highly advantageous in giving extremely rapid response times, high output current densities, remarkable storage stability when wet, and require relatively low operating potentials, giving rise to low background noise, it has been found that such enzyme electrodes are alcohol sensitive, that is to say, they give rise to false readings, and a general increase of background current, in the presence of alcohol, especially ethanol, and thus cannot be used for the reliable determination of analytes, e.g. glucose, in samples containing quantities of alcohol (ethanol).